The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 USC 202) in which the contractor has elected not to retain title.
1. Field of the Invention
The present invention relates to thin-film batteries compatible with integrated circuit manufacturing and in particular to the method for making the same.
2. Description of Related Art
There is an ever-expanding field where solid-state thin-film batteries may be used, including critical space exploration systems. Examples include battery-backed CMOS memory, MEMS switches, and micro sensors. In many of these applications, the ability to integrate a thin film battery directly onto a silicon integrated circuit chip next to the device to be powered would reduce overall device mass and volume as well as open up the possibility of distributing power at the chip level. The ability to deposit high capacity films onto flexible polymer substrates is also appealing.
The ideal thin-film battery has a capacity approaching theoretical levels for the materials system selected, minimal capacity loss over thousands of cycles, and materials and process steps that are compatible with common silicon-based fabrication methods.
In the recent past, RF magnetron sputtered LiCoO2 thin films have emerged as a leading candidate for use as the cathode layer in thin film solid state batteries. Films that exhibit a (104) or (101) out-of-plane texture, have grains that are  greater than 100 nanometers in diameter, and display little to no lattice strain and are most efficient at lithium intercalation and electronic conduction. It has been shown elsewhere that an annealing step of typically 700xc2x0 C. induces the desired film qualities. Because this heating step is too high for many desirable substrates, such as flexible polymer materials or silicon wafers with integrated CMOS devices, an effort has been undertaken to examine the possibility of creating LiCoO2 films with the desired material qualities without using a high temperature process step. To reach this goal, it is critical to understand the deposition parameters that affect film crystallography and composition of LiCoO2 when sputtered at room temperature.
A range of compositions have been observed in sputtered LiCoO2. The Li/Co ratio has been reported to be 0.88 [9] or 1.0+/xe2x88x920.1 when sputtered using a sputter gas consisting of 100% Argon. If sputter gases comprise Ar:O2 mixes of 1:10 and 1:1, the Li/Co ratio was found to be 0.8+/xe2x88x920.08 and 1.15+/xe2x88x920.02, respectively. Concurrently, O/Co ratios ranging from 2.7 to 2.2 have been reported as well. In all cases, these films were found to be amorphous (per x-ray diffraction analysis) if deposited at room temperature and subsequently developed primarily a (003), (101), or (104) out-of-plane texture upon annealing at temperatures in excess of 600xc2x0 C., depending on thickness.
In contrast to these results, preliminary experiments show that it is possible to deposit nano-crystalline LiCoO2 films at room temperature. Some of these films have strong (104) out-of-plane textures and are found to have promising electrochemical properties.
The highest capacity thin film cathode layers (LiCoO2) typically require an annealing step of 700xc2x0 C. Since this high temperature is not compatible with silicon device technology or flexible polymer substrates, the development of a low process temperature ( less than 300xc2x0 C.) cathode layer is needed. LiCoO2 thin-films were RF sputter deposited and subsequently incorporated into thin film batteries. A variety of deposition and post-deposition parameters were varied in an effort to optimize film microstructure and content. Film composition and microstructure were examined using a variety of techniques including x-ray diffraction using synchrotron radiation. It was found that LiCoO2 could be deposited at room temperature in a nano-crystalline state with a strong (104) out of plane texture and a high degree of lattice distortion. By heating these layers to 300xc2x0 C., the grain size is significantly increased while lattice distortion is eliminated. Cycling data reveals that the heating step increases cell capacity to near theoretical values (at lower discharge currents) while significantly improving both the rate capability and discharge voltage.
Accordingly, it is an object of the present invention to provide a thin film battery that has near-theoretical capacities and good discharge capabilities without exceeding temperatures of approximately 300xc2x0 C. and are compatible with silicon processing techniques, which was heretofore a major problem.
Another object of the present invention is to provide a method for growing films with nano-crystalline grains oriented in the proper crystallographic direction at room temperature.
Yet another object of the present invention is to provide a simplified method for manufacturing thin-film batteries by heating the films to 300xc2x0 C. in order to create cathode layers that have near state of the art performance.
These and other objects, which will become apparent as the invention is described in detail below, are provided by a process for making thin-film batteries including the steps of cleaning glass or a silicon substrate having an amorphous oxide layer several microns thick; defining with a mask the layer shape when depositing cobalt as an adhesion layer and platinum as a current collector; using the same mask as the preceding step to sputter deposit LiCoO2 in an argon-oxide gas on the structure while rocking it back and forth; heating the substrate to 300xc2x0 C. for 30 minutes; sputtering with a new mask that defines the necessary electrolyte area; evaporating lithium metal anodes using an appropriate shadow mask; and, packaging the cell in a dry-room environment by applying a continuous bead of epoxy around the active cell areas and resting any thin protective (i.e., insulating) over the top thereof.
Still other objects, features and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein it is shown and described only the preferred embodiment of the invention, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive, and what is intended to be protected by Letters Patent is set forth in the appended claims. The present invention will become apparent when taken in conjunction with the following description and attached drawings, wherein like characters indicate like parts, and which drawings form a part of this application.